Fixes #941 Remove unused step in plan select modal

[remyperona]

Description

Fixes #941

Remove unused step in plan select modal, to go directly from plan select to the checkout page

Type of change

• Bug fix (non-breaking change which fixes an issue).

Detailed scenario

• Display the upgrade plan modal
• Select a plan
• next page is the checkout page

Technical description

Documentation

Removed a lot of unused code, related to the old step + one time plans. Also removed unnecessary request to get the number of images optimized and related data.

Update the JS code to go to the appropriate checkout page depending on the plan and period selected.

Mandatory Checklist

Code validation

• I validated all the Acceptance Criteria. If possible, provide screenshots or videos.
• I triggered all changed lines of code at least once without new errors/warnings/notices.
• I implemented built-in tests to cover the new/changed code.

Code style

• I wrote a self-explanatory code about what it does.
• I protected entry points against unexpected inputs.
• I did not introduce unnecessary complexity.

Unticked items justification

No code coverage possible for the code changed in this PR